For patients with chronic eczema or those with rashes that do not respond well to therapy, patch testing may be indicated. This diagnostic test provides information regarding possible sources of an allergic or irritant contact dermatitis.
For this diagnostic test, large pieces of medical tape with chemicals applied to small disks are placed on the back of unaffected skin. The patches remain on the person for 2 days (48 hours) and removed. Sensitivities to the chemical agents are identified at that time, and patients are asked to return in 1-3 days for a 2nd reading. Showers and baths are not permitted during the patch test, but patients are allowed to sponge bathe. At that time, literature is provided to the patient regarding his/her sensitivities, and the patient is instructed to avoid these chemicals.
Patch testing is different from allergy prick testing. Allergy testing allows the physician to identify causes of allergies, including foods, dust, mites, grass, etc. Patch testing, on the other hand, helps identify causes of rashes. For example, hair dressers are often sensitive to paraphenylenediamine, a substance used in hair dyes; atopic dermatitis patients are typically sensitive to nickel. At our office we use the True Test brand patch test kits, the only FDA approved patch tests, which can detect allergy to 36 different chemicals. However, some patients will need more extensive patch testing and can be provided with a referral to an academic center that does comprehensive patch testing.